1. Field of the Invention
The present invention relates to an optical diffusing plate that gives an anisotropic scattering of a linearly polarized light and has an excellent diffusion property in a scattering direction, and that is suitable for improving visibility, brightness etc. of a liquid crystal display etc. Moreover, the present invention relates to an optical element using the optical diffusing plate concerned.
2. Description of the Prior Art
Conventionally, an optical diffusing plate designed so that anisotropic scattering might be given to a linearly polarized light by making a domain with a refractive index anisotropy contained in a dispersed state in a base material has been known. As the optical diffusing plate concerned; an optical diffusing plate that comprises a combination of a thermoplastic resin and a low molecular weight liquid crystal, that comprises a combination of a low molecular weight liquid crystal and a photo-curable low molecular weight liquid crystal and that comprises a combination of a polyvinyl alcohol and a low molecule liquid crystal, have been known (U.S. Pat. No. 2,123,902 specification, WO 87/01822 official gazette, JP-A 9-274108).
What is expected in the above-mentioned optical diffusing plate is that supplying a linearly polarized light in a state difficult to be absorbed by a polarizing plate reduces an absorption loss, and consequently the brightness of a liquid crystal display is improved. It is expected using this method that the following conventional problems may be solved; a problem based on a large wavelength dependability of a cholesteric liquid crystal in the conventional absorption loss reduction systems in which a cholesteric liquid crystal layer and a quarter wavelength plate are used, and especially, problems that transmitted light is chromic from oblique direction and that these cannot be applied to a reflected type liquid crystal display etc. However, in the above-mentioned conventional optical diffusing plate, there were problems of a difficulty in manufacturing, and of a practical usability in difficult handling with a poor functional stability, when applied in a liquid crystal display, etc.
As an optical diffusing plate in which the above-mentioned problem was solved, an optical diffusing plate using a birefringent film that contains minute domains with different birefringent characteristics in a dispersed state is proposed (JP-A 2000-187105). When this optical diffusing plate is used, the above-mentioned problem is solved and moreover excellent polarization characteristics may be demonstrated. However, in existing optical diffusing plates in which a birefringent film containing a minute domain in a dispersed state is used, each of the above-mentioned minute domain is aligned in the stretching direction of a stretched birefringent film, and a minute domain perpendicularly aligned to the stretching direction is not obtained.
An object of the present invention is to provide an optical diffusing plate with excellent thermal and chemical stability and practical usability, using a birefringent film containing minute domains in a dispersed state having different birefringent characteristics, easily manufactured, wherein a linearly polarized light that may reduce an absorption loss by a polarizing plate may be supplied and an improvement in brightness is expected and at the same time a chromatic problem may not be induced, and the optical diffusing plate is applicable to a reflected type liquid crystal display etc., further wherein a minute domain is aligned perpendicularly to the stretching direction of a stretched birefringent film. And another object of the present invention is to provide an optical element using the above-mentioned optical diffusing plate and further a-liquid crystal display using the same.
As a result of repeated examinations carried out wholeheartedly by the present inventors to solve the above-mentioned problems, it was found out that the above-mentioned object might be attained and the present invention was completed using an optical diffusing plate shown below.
Namely, the present invention relates to an optical diffusing plate comprising a birefringent film and a minute domain with a birefringent characteristic different from the birefringent film in a dispersed state in the birefringent film, wherein the birefringent film comprises a birefringent stretched film, and the minute domain comprises a positive uniaxial liquid crystal polymer, further wherein a length of the minute domain in a direction of stretching axis-is longer than a length in a direction orthogonal to the stretching axis, and the liquid crystal polymer is aligned perpendicularly to a stretching axis of the birefringent stretched film.
Since the minute domain and the birefringent stretched film that contains the minute domain in a dispersed state is formed by a polymer material, the resulting forming material has an excelled handling and is easily manufactured using the optical diffusing plate of the present invention. Moreover, since the forming material has a thermal and chemical stability, it demonstrates stable optical function and excels also in practicality. Moreover, the minute domain is formed by a positive uniaxial liquid crystal polymer and the liquid crystal polymer is perpendicularly aligned on an interface to a base material polymer, and therefore a state is given in which the liquid crystal polymer as a whole is aligned perpendicularly to the stretching axis of the birefringent stretched film because this minute domain is extended in a direction of stretching axis based on stretching of a base material film. Such a state is preferable at a point that a control of a refractive index in a three-dimensional direction is attained unlike in the birefringent film described in the publication of JP-A 2000-187105.
In the above-mentioned optical diffusing plate, it is preferable that the positive uniaxial liquid crystal polymer is a side chain type liquid crystal polymer comprising a monomer unit (a) containing liquid crystalline fragment side chain and a monomer unit (b) containing non-liquid crystalline fragment side chain.
In the above-mentioned optical diffusing plate, a minute domain is distributed in a dispersed state caused by phase separation, and it is preferable that the length in the direction of stretching axis of the minute domain is from 0.05 to 500 xcexcm.
Although the above-mentioned optical diffusing plate may be used as one layer, two or more layers may be used in laminated state so that the xcex94nl direction of an upper layer and a lower layer may have a mutually parallel relationship in order to increase polarization characteristics.
Moreover, the present invention relates to an optical element comprising a laminated layer of at least one kind selecting from a polarizing plate and a retardation plate, and the above-mentioned optical diffusing plate.
Furthermore, the present invention relates to a liquid crystal display comprising the above-mentioned optical diffusing plate or the above-mentioned optical element on one side or both sides of liquid crystal cells.
When the above-mentioned polarizing plate is used, an amount of the transmitted linearly polarized light is increased and an absorption loss is reduced simultaneously, and the brightness of a transmitted type liquid crystal display etc. may be enhanced. Moreover, a possible chromatic problem based on high wavelength dependability as in cholesteric liquid crystal becomes rarely to be induced. Furthermore, the above-mentioned polarizing plate is easily-applicable also in a reflected type liquid crystal display etc., and a liquid crystal display with outstanding brightness and visibility may be stably obtained.